U.S. patent application number 12/542666 was filed with the patent office on 2011-02-17 for nanocomposite support materials.
Invention is credited to Arijit Bose, Christopher Brooks, Vijay T. John, Ganapathiraman Ramanath, Jayashri Sarkar.
Application Number | 20110039692 12/542666 |
Document ID | / |
Family ID | 43588934 |
Filed Date | 2011-02-17 |
United States Patent
Application |
20110039692 |
Kind Code |
A1 |
Brooks; Christopher ; et
al. |
February 17, 2011 |
Nanocomposite Support Materials
Abstract
The present teachings are directed toward hexagonally patterned
porous titania synthesized from a titanium isopropoxide precursor
using a viscous template of surface-active agents separating
nanoscopic bicontinuous channels of water and isooctane. Subsequent
catalyst metal salt reduction in the aqueous nanochannels deposits
well-separated catalyst metal nanoparticles on the pore surfaces.
These nanocomposites exhibit significantly higher carbon monoxide
oxidation efficiency than that obtained with known supports with
higher specific surface area; efficiency is believed to be due to
decreased mass transfer resistance provided the presently disclosed
support material.
Inventors: |
Brooks; Christopher;
(Dublin, OH) ; Bose; Arijit; (Lexington, MA)
; Sarkar; Jayashri; (Columbus, OH) ; Ramanath;
Ganapathiraman; (Schenectady, NY) ; John; Vijay
T.; (Destrehan, LA) |
Correspondence
Address: |
Capitol City TechLaw, PLLC
113 S. Columbus St., Suite 302
Alexandria
VA
22314
US
|
Family ID: |
43588934 |
Appl. No.: |
12/542666 |
Filed: |
August 17, 2009 |
Current U.S.
Class: |
502/339 ;
502/439; 977/700 |
Current CPC
Class: |
B01J 21/063 20130101;
B01J 37/16 20130101; B01D 2255/1021 20130101; B01D 2255/92
20130101; B01J 23/42 20130101; B01J 35/1066 20130101; B01J 35/1061
20130101; B01J 23/74 20130101; B01D 2255/802 20130101; B01J 35/0053
20130101; B01J 37/033 20130101; B01D 53/864 20130101; B01J 35/0066
20130101; B01J 21/066 20130101; B01D 2257/404 20130101; B01J
37/0018 20130101; B01J 21/08 20130101; B01J 23/40 20130101; B01J
21/06 20130101; B01J 35/109 20130101; B01D 53/885 20130101; B01D
2257/502 20130101; B01J 35/006 20130101; B01J 21/04 20130101; B01J
23/52 20130101; B01D 2255/20707 20130101 |
Class at
Publication: |
502/339 ;
502/439; 977/700 |
International
Class: |
B01J 23/42 20060101
B01J023/42; B01J 32/00 20060101 B01J032/00 |
Claims
1. A method of preparing a support material composition comprising:
providing a first organic phase; providing a second organic phase;
contacting the first and second organic phases to form a mixed
organic phase; providing an aqueous phase; contacting and mixing
the mixed organic phase and the aqueous phase to form a reaction
mixture having an interface between the mixed organic phase and the
aqueous phase; allowing a solid to precipitate; heating the
reaction mixture to remove the liquids; and isolating a support
material having macropores, wherein the first organic phase
comprises a first non-polar, non-aqueous solvent and at least one
surface-active agent, the second organic phase comprises a second
non-polar, non-aqueous solvent and at least one support metal
precursor, and the support material comprises oxides of the support
metal precursors.
2. The method according to claim 1, wherein the first and second
non-polar, non-aqueous solvents independently comprise one or more
members selected from the group consisting of a liquid hydrocarbon,
pentane, hexane, heptane, isooctane and octane.
3. The method according to claim 1, wherein the surface-active
agent comprises at least one single or double-tailed cationic,
anionic or non-ionic component.
4. The method according to claim 1, wherein the surface-active
agent comprises at least one or more members selected from the
group consisting of AOT, lecithin, oleic acid, oleyl amine,
trioctylphosphine oxide, trioctyl phosphine, stearic acid,
cetyltrimethylammonium bromide, polysorbate 80, and a mixture of
polyoxyethylene tert-octylphenyl ethers.
5. The method according to claim 1, wherein the surface-active
agent comprises AOT.
6. The method according to claim 1, wherein the support metal
precursor comprises an alkoxide of one or more metals selected from
the group consisting of silicon, titanium, zirconium and
aluminum.
7. The method according to claim 6, wherein the alkoxide comprises
an at least one member selected from the group consisting of
silicon isopropoxide, titanium isopropoxide, zirconium
isopropoxide, and aluminum isopropoxide.
8. The method according to claim 1, wherein heating the reaction
mixture comprises exposing the reaction mixture to temperatures
greater than about 550.degree. C.
9. The method according to claim 1, wherein the contacting occurs
at room temperature.
10. The method according to claim 1, wherein the diameters of the
macropores of the support material increase with increasing water
content.
11. The method according to claim 1, wherein the ratio of water to
surface-active agent controls the size of the diameters of the
macropores of the support material.
12. The method according to claim 1, wherein the aqueous phase
further comprises a second metal salt.
13. The method according to claim 12, wherein the second metal salt
comprises one or more members selected from the group consisting of
iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum and
gold.
14. A method of preparing a supported catalyst composition
comprising providing a first organic component comprising a
non-polar, non-aqueous solvent, a support metal precursor, and
least one surface-active agent; providing an aqueous component
comprising a catalyst metal precursor; contacting the aqueous
component with the first organic component to form an emulsion
phase; providing a reducing agent; contacting the reducing agent
with the emulsion phase to form a reaction mixture; heating the
reaction mixture to remove any liquid components; and isolating the
supported catalyst composition, wherein the supported catalyst
composition comprises oxides of the support metal arranged in a
structure having both macropores and mesopores with particles of
the catalyst metal dispersed thereon.
15. The method according to claim 14, wherein the non-polar,
non-aqueous solvent comprises one or more members selected from the
group consisting of a liquid hydrocarbon, pentane, hexane, heptane,
isooctane and octane.
16. The method according to claim 14, wherein the surface-active
agent comprises at least one single or double-tailed cationic,
anionic or non-ionic component.
17. The method according to claim 14, wherein the surface-active
agent comprises at least one or more members selected from the
group consisting of AOT, lecithin, oleic acid, oleyl amine,
trioctylphosphine oxide, trioctyl phosphine, stearic acid,
cetyltrimethylammonium bromide, polysorbate 80, and a mixture of
polyoxyethylene tert-octylphenyl ethers.
18. The method according to claim 14, wherein the surface-active
agent comprises AOT.
19. The method according to claim 14, wherein the support metal
comprises an alkoxide of one or more metals selected from the group
consisting of silicon, titanium, zirconium and aluminum.
20. The method according to claim 17, wherein the alkoxide
comprises an at least one member selected from the group consisting
of silicon tetramethoxide, silicon isopropoxide, titanium
isopropoxide, zirconium isopropoxide, and aluminum
isopropoxide.
21. The method according to claim 14, wherein heating the reaction
mixture comprises exposing the reaction mixture to temperatures
greater than about 550.degree. C.
22. The method according to claim 14, wherein the contacting steps
occur at room temperature.
23. The method according to claim 14, wherein the diameters of the
macropores of the support material increase with increasing water
content.
24. The method according to claim 14, wherein the ratio of water to
surface-active agent controls the size of the diameters of the
macropores of the support material.
25. The method according to claim 14, wherein the catalyst metal
comprises one or more members selected from the group consisting of
iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum and
gold.
26. A supported catalyst comprising a support material with
nanosized channels and pores, a catalyst metal component supported
on the support material, and wherein the nanosized channels and
pores are formed by deposition of the support material from an
organic phase comprising a support material precursor, and the
catalyst metal component is deposited from an aqueous phase
comprising a water soluble catalyst metal precursor.
27. The supported catalyst according to claim 26, wherein the
support material precursor comprises an alkoxide of one or more
metals selected from the group consisting of silicon, titanium,
zirconium and aluminum.
28. The supported catalyst according to claim 26, wherein the
support material precursor comprises titanium isopropoxide.
29. The supported catalyst according to claim 26, wherein the water
soluble catalyst metal precursor comprises metal-containing
compounds of one or more metals selected from the group consisting
of iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum
and gold.
Description
BACKGROUND
[0001] 1. Field of the Invention
[0002] The present teachings relate to support material
formulations, methods for synthesizing same, and their use as
supports for catalytically active material, particularly for CO
oxidation and lean burn deNOx control.
[0003] 2. Discussion of the Related Art
[0004] Noble metal nanoparticles loaded on nanoporous titania
(TiO.sub.2) supports are attractive for catalyzing reactions
germane to energy generation and environmental preservation, for
example, photocatalytic generation of hydrogen from water and
carbon monoxide (CO) oxidation. The efficiency of the TiO.sub.2
supported catalysts is increased through electronic-structure-level
interactions at the support-catalyst metal interface. For example,
the amount of CO converted to CO.sub.2 per unit mass of Pt catalyst
per unit time (referred to as the turnover frequency, hereinafter
"TOF") is hundredfold higher with a TiO.sub.2 support than with a
SiO.sub.2 support over a temperature range of 150 to 550.degree. C.
Clearly, the catalyst and support chemistries, the support
structure, and the catalyst particle distribution within the pores
of the support are all parameters that impact the TOF and should be
controlled to increase TOF.
[0005] Several techniques have been devised to synthesize
catalyst-support composites, such as impregnation, sol-gel based
processes, flame spray synthesis, electrodeposition, laser
pyrolysis, sonochemistry and UV irradiation. In all of these
processes, the support material, typically disordered, is prepared
first, followed by the transfer to, or formation of, catalyst
particles within the pores of the support material. Such sequential
approaches have disadvantages such as pore plugging, insufficient
control of catalyst distribution within the pores, and high costs
due to production complexity. Disordered and disconnected pores in
the support material can lead to increased mass transfer
resistance.
[0006] There is a need for synthesis routes for supported catalyst
that allow for formation of patterned and interconnected porous
supports with catalyst nanoparticles of controllable size
distributed throughout the support structure.
SUMMARY OF THE PRESENT DISCLOSURE
[0007] The present teachings are directed to a technique utilizing
soft templates for the synthesis of highly organized nanoporous
supports, such as anatase, having a uniform dispersion of Pt
nanoparticles. The present teachings include a catalyst composite
where both the support and the catalyst are synthesized using the
same soft template, at room temperature. One feature of the present
teachings is the distribution of the catalyst metal nanoparticles
uniformly throughout the support with minimal agglomeration and
pore blocking
[0008] The process of the present teachings uses the principle of
multi-component synthesis in the same template in the delineated
domains of water and oil. This synthesis scheme exploits the
organization provided by a surfactant, or surface-active agent,
template system. A microemulsion containing a non-polar,
non-aqueous solvent, and at least one surface-active agent, such as
a surfactant and/or an emulsifier, for example, isooctane, dioctyl
sulfosuccinate sodium salt (herein "AOT"), and lecithin upon
addition of water produces a highly viscous bicontinuous `gel`
phase with nearly equal proportions of isooctane and water
distributed as nanochannels. The high viscosity of this surfactant
template phase is believed to be important for immobilizing the
support structure and the precipitated catalyst nanoparticles.
[0009] In one specific example of the presently taught process,
titanium isopropoxide (herein "TIP") can be dissolved in an organic
solvent mixture prior to addition of the aqueous phase. Anatase
titania can be formed by the hydrolysis and condensation of TIP.
Since TIP is soluble in organic solvents, and has limited
solubility in water, the TiO.sub.2 forms at the oil/water
interfaces. The role of the surface-active agents is to organize
the template, and typically they do not participate in the
reaction. Thus, the template remains intact and enables the
TiO.sub.2 to inherit the template microstructure. The samples can
then be solvent dried and calcined using a programmed calcination
procedure.
[0010] The present disclosure includes a method of preparing a
support material composition by providing a first organic phase, a
second organic phase, and an aqueous phase, and contacting the
first and second organic phases to form a mixed organic phase. Then
contacting and mixing the mixed organic phase with the aqueous
phase to form a reaction mixture having an interface between the
mixed organic phase and the aqueous phase, allowing a solid to
precipitate, heating the reaction mixture to remove the liquids,
and isolating the support material. In this present method, the
first organic phase comprises a first non-polar, non-aqueous
solvent, at least one surface-active agent which can be both a
surfactant and/or an emulsifier, the second organic phase comprises
a second non-polar, non-aqueous solvent and a support metal
precursor. The resulting support material comprises oxides of the
support metal precursor and has a macroporous structure.
[0011] Another method disclosed herein includes preparing a
supported catalyst composition by providing a first organic
component containing a non-polar, non-aqueous solvent, a support
metal precursor, and least one surface-active agent, and an aqueous
component comprising water and a catalyst metal precursor. The two
components are contacted to form an emulsion phase. A reducing
agent is provided and then contacted with the emulsion phase to
form a reaction mixture. The reaction mixture is heated to remove
any liquid components, and the supported catalyst composition is
isolated. The supported catalyst composition is made of the oxides
of the support metal arranged in a structure having both macropores
and mesopores with particles of the reduced catalyst metal
dispersed thereon.
[0012] Also disclosed herein is a process for the production of a
supported catalyst material by providing a microemulsion comprising
a first organic phase containing an organic solvent, at least one
surface-active agent, such as an emulsifier, and/or a surfactant,
and also providing a second organic solution containing a support
metal precursor dissolved in the organic solvent. The second
organic phase is contacted with the microemulsion to form an
organic mixture. The organic mixture is then contacted with an
aqueous phase comprising water and a catalyst metal precursor,
which by hydrolysis of the support metal precursor, at the
interface between the organic mixture and the aqueous phases,
produces the oxide of the support metal. This product is then dried
and calcined to produce the supported catalyst material with the
catalyst metal dispersed therein.
[0013] Also disclosed by this application is a supported catalyst
made up of a support material with nanosized channels and pores,
and a catalyst metal component supported on the support material.
The nanosized channels and pores are formed by deposition of the
support material from an organic phase containing a support
material precursor, and the catalyst metal component is deposited
from an aqueous phase including a water soluble catalyst metal
precursor.
[0014] The presently taught synthesis strategy for creating highly
organized composites has wide applications beyond those reported
here, including without limitation, photocatalysis, photonic
crystals, sensors and solar cells assemblies.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The accompanying drawings which are included to provide a
further understanding of the present disclosure and are
incorporated in and constitute a part of this specification,
illustrate various embodiments of the present disclosure and
together with the detailed description serve to explain the
principles of the present disclosure. In the drawings:
[0016] FIGS. 1a through 1c, and 1h are SEM images, 1d through 1f
are TEM images, 1g is an X-ray diffractogram, and inserts a, c and
h are pore diameter distributions determined from SEM images;
[0017] FIGS. 2a through 2g are SEM images, and 2h and 2i are TEM
images of TiO.sub.2 supports;
[0018] FIG. 3a is an SEM micrograph, 3b is a TEM micrograph, and
FIGS. 3c and 3d are core-level spectra measured by XPS;
[0019] FIGS. 4a, 4b and 4c are CO to CO.sub.2 conversion efficiency
graphs, and FIG. 4d is specific surface areas of various
samples;
[0020] FIG. 5 is a representation of a synthetic route according to
the present teachings;
[0021] FIG. 6 is a series of photographs of a synthesis according
to the present teachings;
[0022] FIGS. 7a and 7b are TEM images of a material according to
the present teachings;
[0023] FIGS. 8a through 8c are TEM images of a material according
to the present teachings;
[0024] FIG. 9 is an SEM image of a material according to the
present teachings, and
[0025] FIG. 10 is a series of photographs of a synthesis according
to the present teachings.
DETAILED DESCRIPTION
[0026] The present teachings are directed to a method of preparing
a support material composition by providing a first organic phase,
providing a second organic phase, and contacting the first and
second organic phases to form a mixed organic phase. An aqueous
phase is provided, and then contacted and mixed with the mixed
organic phase to form a reaction mixture. A solid is allowed to
precipitate, the reaction mixture is heated to remove the liquids,
and the support material is isolated. In this present method, the
first organic phase includes a non-polar, non-aqueous solvent, a
surface-active agent, such as a surfactant and/or an emulsifier,
and the second organic phase includes a support metal precursor and
a non-polar, non-aqueous solvent. An interface between the mixed
organic phase and the aqueous phase can be formed in the reaction
mixture. The isolated support material is composed of the oxides of
the support metal.
[0027] The non-polar, non-aqueous solvent utilized in the presently
disclosed method can be a liquid hydrocarbon, for instance,
pentane, hexane, heptane, and octane. One liquid hydrocarbon of
interest is isooctane.
[0028] The presently disclosed method can utilize as a
surface-active agent single and double-tailed cationic, anionic and
non-ionic components. These single and double-tailed cationic,
anionic and non-ionic components can include, for example, one or
more members selected from the group consisting of AOT, lecithin,
oleic acid, oleyl amine, trioctylphosphine oxide ("TOPO"), trioctyl
phosphine ("TOP"), stearic acid, cetyltrimethylammonium bromide
("CTAB"), Triton.RTM. X-100 (a mixture of polyoxyethylene
tert-octylphenyl ethers) and TWEEN.RTM.-80 (polysorbate 80). The
surfactant, AOT, and the emulsifier, lecithin, are of particular
utility in the present methods.
[0029] Within the second organic phase, the support metal precursor
can be a metal alkoxide can be an alkoxide of, for example,
silicon, titanium, zirconium and aluminum. Of particular interest
are metal alkoxides selected from the group consisting of silicon
isopropoxide, titanium isopropoxide, zirconium isopropoxide, and
aluminum isopropoxide. Suitable support metals can include nearly
any metal which can form a stable metal oxide.
[0030] The heating and drying of the reaction mixture comprises
exposing the reaction mixture to temperatures greater than about
550.degree. C., in a ramped fashion to remove any liquid
components. The other contacting steps in the presently disclosed
method can occur at room temperature.
[0031] The diameters of the macropores of the support material
prepared by the presently taught method increase with increasing
water content, more specifically, the ratio of water to the
surface-active agent controls the size of the diameters of the
macropores of the support material. The effect of water to
surface-active agent concentration is discussed below in more
detail, and is illustrated in FIGS. 2a through 2f, in
particular.
[0032] In some embodiments of the presently disclosed method, the
aqueous phase can also contain a second metal salt, typically any
catalytically active metal to be dispersed within and on the
support materials. This second catalyst metal can include one or
more members selected from the group consisting of, for example,
iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum and
gold.
[0033] In one example of the presently disclosed method, the
synthesis of Pt-loaded TiO.sub.2 composites, PtCl.sub.4, a
water-soluble salt precursor, was reduced by sodium borohydride to
form Pt nanoparticles. The even distribution of the metal ions,
here Pt.sup.4+, throughout the gel and the gel's high viscosity can
enhance the ability of the presently disclosed method to produce
well-distributed catalyst metal nanoparticles. The method can
result in uniformly distributed and highly separated Pt
nanoparticles inside the pores of the TiO.sub.2 support.
[0034] The presently disclosed supported catalyst composition can
have a highly organized support structure with a bimodal pore size
distribution (meso and macro) and can have catalyst nanoparticles
of controllable size distributed throughout the support structure.
One feature of the presently disclosed material is the uniform
distribution of the catalyst nanoparticles throughout the support
with minimal agglomeration and pore blocking. The presently
disclosed materials typically perform similar to or better than
their commercial counter parts in terms of CO oxidation and NOx
reduction reactions.
[0035] The catalytic activity of materials prepared by the
presently disclosed methods has been further characterized by
examining the oxidation of CO to CO.sub.2, and the CO conversion
efficiency of the present nanocomposites is significantly better
than a commercial catalyst material with a four-fold higher surface
area. The higher efficacy of the present nanocomposite compositions
is presently attributed, without limiting ourselves to this
reasoning, to the structure of pore patterning which extends
throughout the powder particles, and the pore interconnectivity,
which can provide improved mass transfer of both reactants to and
products from the catalyst sites present in the pores.
[0036] The present disclosure also teaches a method of preparing a
supported catalyst composition comprising the steps of providing a
first organic component containing a support metal component,
providing an aqueous solution containing a catalyst metal, and
contacting the aqueous solution with the first organic component to
form an emulsion phase. A reducing agent is then provided, and
contacted with the emulsion phase to form a reaction mixture, which
is then heated to remove any liquid components, and the supported
catalyst composition is isolated. In this disclosed method, the
first organic component comprises a non-polar, non-aqueous solvent
component, a surface-active agent, such as a surfactant, and/or an
emulsifier, and the support metal component, the aqueous solution
comprises water and the catalyst metal, and the isolated supported
catalyst composition comprises oxides of the support metal arranged
in a structure having both macropores and mesopores with the
catalyst metal particles dispersed thereon.
[0037] This disclosed method can utilize a non-polar, non-aqueous
solvent component which can be a liquid hydrocarbon, for instance,
pentane, hexane, heptane, octane and isooctane.
[0038] The presently disclosed method can utilize as a
surface-active agent single and double-tailed cationic, anionic and
non-ionic components. These components can include, for example,
one or more members selected from the group consisting of AOT,
lecithin, oleic acid, oleyl amine, trioctylphosphine oxide
("TOPO"), trioctyl phosphine ("TOP"), stearic acid,
cetyltrimethylammonium bromide ("CTAB"), Triton.RTM. X-100 (a
mixture of polyoxyethylene tert-octylphenyl ethers) and
TWEEN.RTM.-80 (polysorbate 80). The surfactant, AOT, and the
emulsifier, lecithin, are of particular utility in the present
methods.
[0039] The support metal component, which forms the structure of
the presently taught support for the catalyst, can be an alkoxide
of, for instance, silicon, titanium, zirconium and aluminum.
Specific examples of suitable metal alkoxides can include at least
one member selected from the group consisting of silicon
tetramethoxide, silicon isopropoxide, titanium isopropoxide,
zirconium isopropoxide, and aluminum isopropoxide.
[0040] The reducing agents which can be utilized in the present
method can include, for example, reducing gas atmospheres (for
instance, pure H.sub.2 or a mixture of H.sub.2 in N.sub.2 and/or He
or other inert gases), sodium borohydride, 1,2-hexadecanediol,
trimethylaluminum, formic acid, ammonium hydroxide, hydrazine
monohydrate, sodium metal, potassium metal, sodium naphthalide,
potassium triethyl borohydride, magnesium metal, magnesium
anthracide, and combinations thereof. Sodium borohydride has
demonstrated particular utility in the present methods.
[0041] The presently taught method includes a step of heating the
reaction mixture to dry and calcine the supported catalyst
composition and can include exposing the reaction mixture to
temperatures greater than about 550.degree. C. In contrast, the
other steps of the presently taught method can occur at room
temperature.
[0042] In the presently disclosed method, the diameters of the
macropores of the support material increase with increasing water
content. Typically, by increasing the ratio of water to
surface-active agent, particularly the surfactant, here preferably
AOT, the size of the diameters of the macropores of the support
material can be increased.
[0043] The catalyst metal utilized in the presently taught method
can include one or more members selected from the group consisting
of, without limitation, iron, ruthenium, cobalt, rhodium, nickel,
palladium, platinum and gold.
[0044] Further provided by the present disclosure is a process for
the production of a supported catalyst material including the steps
of providing a microemulsion comprising an organic phase containing
an organic solvent, at least one surface-active agent, and
preferably, both an emulsifier and a surfactant, and also providing
an organic solution containing a support metal precursor dissolved
in an organic solvent. Next, the organic solution is contacted with
the microemulsion to form an organic mixture. Then, an aqueous
phase comprising water and a catalyst metal precursor is provided,
and contacted with the organic mixture to produce a product of an
oxide of the support metal by hydrolysis and condensation reaction
of the support metal precursor at the interface between the organic
mixture and the aqueous phases. The support metal oxide is then
dried and calcined to produce the supported catalyst material.
[0045] A supported catalyst is also disclosed herein. The presently
taught supported catalyst includes a support material with
nanosized channels and pores, and a catalyst metal component
supported on the support material. For this supported catalyst, the
nanosized channels and pores are formed by deposition of the
support material from an organic phase comprising a support
material precursor, and the catalyst metal component was deposited
from an aqueous phase comprising a water soluble catalyst metal
precursor. Preferably, these depositions occur at room
temperature.
[0046] The support material precursor can include one or more
members selected from the group consisting of Si, Ti, Al, or Zr
alkoxides. Some suitable support material precursor include
titanium isopropoxide and silicon tetramethoxide. The water soluble
catalyst metal precursor can include one or more member selected
from the group consisting of nitrates, halides, such as the
chlorides, of metals, such as, iron, ruthenium, cobalt, rhodium,
nickel, palladium, platinum and gold.
[0047] The presently disclosed method can be utilized to produce
TiO.sub.2-containing support material with highly organized pore
structures having a bimodal pore size distribution in the 3 to 15
nm range and the 20 to 250 nm range. As used herein, "macropore"
refers to pores having diameters ranging from between about 20 to
about 250 nm, and "mesopore" refers to pores having diameters
ranging from between about 3 to about 15 nm. The present
definitions of "macro" and "meso" are selected in order to clearly
differentiate between the larger and smaller pore sizes of the
bimodal pore distribution; the IUPAC definitions would not permit
such differentiation. Such materials can find tremendous
application in the reforming and similar catalysis areas.
[0048] The methods and compositions of the present teachings are
further explained with reference to the Examples and Figures set
forth below and herein. TiO.sub.2 supports were synthesized from a
soft-template using isooctane, AOT, lecithin, TIP and water
according to the presently disclosed method as set forth in Example
1 below. The white powder product consists of particles with pore
diameters between 20 nm and 250 nm organized with hexagonal
symmetry similar to the surfactant template. FIG. 1a shows a SEM
micrograph of a powder particle from the sample prepared using a
template that had a W.sub.0 equal to 70. The average macropore
diameter, obtained from the distribution, shown in the inset in
FIG. 1a, is about 100 nm. Due to the inheritance of the underlying
bicontinuous surfactant template morphology, the macropores are
interconnected through the regions marked by the arrows in FIG. 1b.
A higher magnification image, shown in FIG. 1c, reveals the
mesopores (see arrows). The average size of the mesopores,
determined from the SEM images, is about 5 nm. BET pore diameter
distribution for the same support obtained from a 55 point analysis
shows the pore diameter has essentially a bimodal distribution with
maxima at 3 nm and 5.5 nm, respectively, consistent with the
mesopore diameter measured using SEM. The hierarchical
interconnected porous structures with bimodal pore diameter
distribution can be conducive for improved access to the surfaces,
including catalyst sites, inside the support for both reactants and
products.
[0049] Thin-section transmission electron microscope (TEM) images
of the sample from lateral and axial cross-sections, shown in FIGS.
1d and 1e, confirm that the TiO.sub.2 inherits the hexagonal
template morphology. Solvent removal and calcination is understood
to break the long-range crystalline symmetry that existed in the
original surfactant template, but the essentially hexagonal
patterning is maintained. Images taken from a range of
thin-sections taken from different portions of materials prepared
according to the present teachings consistently show this
structure, and are therefore assumed to persist throughout. The
breaks in the pore walls shown by the arrows in FIG. 1e confirm
macropore interconnectivity inside the support. Higher
magnification TEM images reported in FIG. 1f show that the
macropore walls are comprised of an assembly of fused TiO.sub.2
nanoparticles.
[0050] Energy dispersive X-ray spectroscopy has shown prominent Ti
and O peaks corresponding to a TiO.sub.1.76 stoichiometry, with
only trace amounts (less than 1.8 atomic %) of P, S and Na
observed, suggesting that contamination of the pores from residual
surface-active agents is negligible. The powder X-ray
diffractogram, shown in FIG. 1g, reveals that the TiO.sub.2 in the
presently taught material is anatase, in agreement with earlier
work showing that anatase is preferred at temperatures below
550.degree. C. The peaks are broad, indicating that the crystallite
size in the support is small. The very small peaks, at about
33.degree. 2.theta., are due to the presence of trace quantities of
Na.sub.2SO.sub.4 left from the AOT surfactant after the calcination
procedure. These elements were also identified in the EDS
spectra.
[0051] The characteristic macropore diameters in the presently
disclosed TiO.sub.2 supports prepared according to the present
methods are about tenfold larger than that of the template water
channels determined using small angle neutron scattering ("SANS")
in the bicontinuous surfactant templates that contained no TIP.
Cryogenic scanning electron microscope ("cryo-SEM") images of
TiO.sub.2 samples prior to solvent removal, shown in FIG. 1h,
reveal macropore diameters between about 20 nm to 150 nm, which are
intermediate between the template feature sizes obtained by SANS
and pore diameters of the calcined TiO.sub.2 support measured by
SEM and TEM.
[0052] According to our present theory without being limited
thereto, this observation implies that one major contributing
factor to the pore diameter increase is solvent evaporation, which
can consolidate the TiO.sub.2 nanoparticles into the walls by
capillary forces. The solubility of AOT in TIP is about 0.4 g/ml.
The preferential dissolution of AOT into the TIP/isooctane solution
leaves excess lecithin at the aqueous/organic interfaces. Since
lecithin by itself forms water pools of larger diameter in
water-in-oil microemulsions, an additional factor for the observed
pore diameter can be this selective loss of AOT from the
isooctane/water interface. Examination of microstructures prior to
and after calcination reveals only a 3% increase in macropore
diameter, which is small compared to the solvent evaporation and
surfactant dissolution effects.
[0053] In the presently disclosed methods, the mean pore diameter
of the support structures can be tuned, without altering the
qualitative features of the TiO.sub.2 microstructure, by adjusting
the water content. The effect of increasing the water content from
W.sub.0=70 to 200, shown in FIGS. 2a through 2f, results in an
increase in average macropore diameter from about 100 nm to about
185 nm. The internal macropore structure remains basically
unaffected with increasing W.sub.0, but the macropore wall
thickness increases. This effect is believed to be due to the
additional water driving the TIP hydrolysis to completion. The BET
surface area of the supports also increases marginally as the water
content is increased, resulting in BET's of 24.4, 34.2, 37.6 and
41.9 m.sup.2/g for W.sub.0=70, 100, 150 and 200, respectively.
[0054] According to another embodiment of the presently disclosed
methods, titania support synthesis can be carried out in the
presence of a metal salt, such as PtCl.sub.4, and then reduced in a
subsequent step by adding a reducing agent, such as sodium
borohydride, and TiO.sub.2 supports decorated with 3-5 nm diameter
Pt nanoparticles are produced. The Pt nanoparticles are
well-separated and uniformly distributed on the pore surfaces, as
seen in FIGS. 3a and 3b. The high viscosity (zero shear viscosity
of about 10.sup.5 poise) of the underlying template can immobilize
the Pt nanoparticles inside the aqueous nanochannels. The hexagonal
patterning of the pores is preserved, indicating that the Pt salt
reduction does not disrupt pore organization in the support. EDS
spectra from the nanocomposite samples show prominent Ti, O and Pt
peaks, along with trace amounts (less than 1.8 atomic %) of P and S
from the surfactants (data not shown). The specific BET surface
areas for the nanocomposites are 22.3, 26.9, and 36.9 m.sup.2/g for
samples of W.sub.0=70, 100 and 200, respectively. The surface area
of the support is essentially unaffected by the addition of Pt in
the support.
[0055] X-ray photoelectron spectroscopy (XPS) scans in the vicinity
of the Ti 2p and Pt 4f core-level bands are shown in FIGS. 3c and
3d. The Ti 2p.sub.3/2 band at 458.5 eV corresponds to the Ti.sup.4+
state, while the Pt 4f.sub.7/2 band at 71.1 eV is in good agreement
with the Pt.sup.0 state. The reaction products in the presently
taught supported catalyst composition are thus titania and metallic
platinum.
[0056] The steady state conversion for carbon monoxide oxidation to
carbon dioxide using unwashed Pt/TiO.sub.2 nanocomposites was
measured for samples prepared according to the presently taught
methods with varying W.sub.0 (70, 150 and 200). These samples had
Pt loadings of 0.5%, 1.5% and 1.8% by weight, respectively. CO
oxidation was examined over a temperature range of 70.degree. C. to
250.degree. C. As shown in FIG. 4a, the conversion reached a
maximum at about 150.degree. C., and the extent of CO conversion
increased with increased Pt loading as expected.
[0057] FIG. 4b shows that increasing aqueous content, which
increases the specific surface area of the support, does not appear
to a very significant effect on the activity of the composites.
Here, each composite had an overall Pt loading of 1.7 wt. %, and
were washed prior to undergoing CO conversion testing. The
presently disclosed methods and compositions can result in
materials having highly interconnected, patterned samples, and
apparently a doubling in the specific surface area does not result
in additional catalyst sites being exposed to the reactants.
[0058] The catalytic activity of washed and unwashed Pt-containing
nanocomposites according to the present teachings was compared to
that of a commercially available TiO.sub.2 support. XT 25376,
available from St. Gobain-Norpro, was wet impregnated to a Pt
loading of 1.8 wt. %. The unwashed material according to the
present teachings performed equivalently despite having a four-fold
lower specific surface area than the commercial sample (see FIG.
4c).
[0059] Additionally, there is a marked difference in the
performance of washed and unwashed samples of the presently taught
catalyst compositions. The results for samples loaded at 1.8 wt. %
Pt are shown in FIG. 4c. The CO conversion versus temperature curve
shifts noticeably towards lower temperatures, reaching a maximum
conversion at about 100.degree. C. for the washed sample compared
to about 150.degree. C. for the unwashed one.
[0060] Chemisorption studies were also conducted on these three
samples described above. The active catalyst surface area for the
commercial sample was 102.20 m.sup.2/g Pt with a 41.37% metal
dispersion. The unwashed sample had an active catalyst surface area
of 58.97 m.sup.2/g Pt with a 23.87% metal dispersion whereas, after
washing, the nanocomposite had an active surface of 134.76
m.sup.2/g Pt with a 54.56% metal dispersion. The presently
disclosed synthesis methods can produce small well-distributed Pt
nanoparticles. The NaCl produced as a byproduct in one of the
reaction steps can be removed by washing, allowing a higher active
surface area and a better Pt dispersion.
[0061] The higher efficacy in the catalyst compositions according
to the present teachings is believed to be due to lower mass
transfer resistance for the reactants, as well as the products, in
the highly organized interconnected macro- and meso-pores and the
well-distributed discrete Pt nanoparticles. It is known that an
enhanced effective mass diffusivity occurs from a patterned highly
connected porous network as opposed to a more random network. See
Mezedur, M. M.; Kaviany, M., and Moore, W., "Effect of pore
structure, randomness and size on effective mass diffusivity,"
AIChE Journal, Vol. 48, pp. 15-24 (2002). Here, with the presently
taught methods, even an unwashed sample of the presently taught
composition shows an advantage over the commercial sample, and
washing to remove salts results in improved performance.
[0062] All publications, articles, papers, patents, patent
publications, and other references cited herein are hereby
incorporated herein in their entireties for all purposes.
[0063] Although the foregoing description is directed to the
preferred embodiments of the present teachings, it is noted that
other variations and modifications will be apparent to those
skilled in the art, and which may be made without departing from
the spirit or scope of the present teachings.
[0064] The following examples are presented to provide a more
complete understanding of the present teachings. The specific
techniques, conditions, materials, and reported data set forth to
illustrate the principles of the present teachings are exemplary
and should not be construed as limiting the scope of the present
teachings.
EXAMPLES
Experimental
[0065] Isooctane (herein "10"), AOT, TIP, platinum chloride
(PtCl.sub.4) and sodium borohydride (NaBH.sub.4) were obtained from
Sigma Aldrich. Lecithin (L-.alpha. phosphatidylcholine, 95% plant
Soy) was used as received from Avanti Polar Lipids.
[0066] W.sub.0 refers to the molar ratio of water to the
surface-active agent(s), generally. For the Examples presented
herein, only the surface-active agent AOT is utilized in the
calculation of the W.sub.0 ratio. For instance, W.sub.70 refers to
a reaction having 70 moles of water to 1 mole of AOT present in the
reaction mixture. As used herein, room temperature refers to a
temperature ranging from 20 to 25.degree. C. (68 to 77.degree.
F.)
Microanalytical Characterization
[0067] A Hitachi S-4800 Field Emission SEM was used to characterize
the support microstructure and nanoparticle dispersions. An Oxford
INCA system was used for the EDS elemental analysis. Thin sections
of samples for TEM measurements were prepared using a MT2-B DuPont
Ultramicrotome by embedding the powder particles into an epoxy
resin, curing overnight and cutting 70-90 nm thick slices using a
diamond knife. TEM images were obtained in a JEOL 1200 EX
instrument operated at 120 kV. A Bruker D8 Advanced X-Ray
diffractometer was used for phase identification. Core-level
spectra from the samples were obtained by XPS using a PHI 5400
instrument with a Mg K.alpha. source. The spectra were collected
using a pass energy of 23.5 eV, and corrected for charging by using
the adventitious carbon is peak at 285 eV.
[0068] Hydrogen chemisorption on a Micrometrics AutoChem 2910 was
used to obtain the active metal surface area and dispersion of Pt.
Hydrogen reduction was first used to prepare the samples.
Approximately 0.5 g of sample was loaded for each experiment and
reduced in situ in a 10% H.sub.2/90% He mix at 300.degree. C. for 1
hr. The flow was then stopped and the sample allowed to degas for 3
hrs at 300.degree. C. and then brought to room temperature. The
sample was then introduced to Ar carrier at 50 ml/min, and the
temperature was ramped to 50.degree. C. at 5.degree. C./min to
purge any reduction products.
[0069] For pulse chemisorption, H.sub.2 was introduced using a 10%
H.sub.2-90% Ar mixture with 100% Ar as the background carrier gas.
100 .mu.L of the mixture was dosed every 3 minutes until no further
hydrogen was chemisorbed, and the amount of adsorbed H.sub.2 was
calculated. The Pt specific surface area (surface area of Pt/gm of
Pt) and dispersion were calculated assuming a H.sub.2/Pt
stoichiometry of 2.
Example 1
Porous Titania Synthesis
[0070] TIP was added in a 1:1 volume ratio with isooctane to a
solution of isooctane/AOT (0.8M)/lecithin (0.4M). A calculated
amount of water, corresponding to the desired W.sub.0, was added to
this solution, and mixed using a vortex mixer. Immediately after
water addition, a white precipitate was observed indicating
TiO.sub.2 formation.
[0071] The samples were dried at 60.degree. C. for 24 hrs and
calcined by ramping the temperature in increments of 50.degree. C.
every 30 min, starting at 400.degree. C. and going to 550.degree.
C. The sample was left at 550.degree. C. for 4.5 hr to obtain a
white powder. The calcination step is believed to eliminate any
trace amounts of surfactants left after solvent removal. This
synthesis scheme is presented in FIGS. 5 and 6, and various SEM
images of the resulting product are presented in FIGS. 7, 8 and
9.
Example 2
Pt-Loaded Titania Nanocomposite Synthesis
[0072] An aqueous solution (0.015M) of PtCl.sub.4 was added to a
solution of TIP/isooctane/AOT/lecithin to reach a desired W.sub.0.
Then a 0.1 M NaBH.sub.4 solution was added to reduce the
PtCl.sub.4. A slight excess of reducing agent was used to ensure
complete reduction of the Pt.sup.4+ ions. The sample underwent a
color change from a light yellow to black, indicating the reduction
of Pt.sup.4+ ions to Pt metal. The samples were then dried at
60.degree. C. for 24 hrs and calcined by ramping the temperature in
increments of 50.degree. C. every 30 min, starting at 400.degree.
C. and going to 550.degree. C. The sample was left at 550.degree.
C. for 4.5 hr to obtain the final sample. This synthesis scheme is
illustrated in FIG. 10.
[0073] Small quantities of NaCl are produced from the sodium ions
formerly associated with the reducing agent and the chloride ions
formerly associated with the platinum salt. The presence of NaCl
within the pores can block some active sites on the catalyst. A
number of samples were washed with distilled deionized water five
times after calcination to remove any NaCl present therein.
[0074] Samples of the Pt-loaded titania nanocomposites according to
the present teachings were prepared at various W.sub.0 levels
including 70, 100, 130, 150, 170 and 200, and at various Pt
loadings (wt. % Pt) including 0.5, 1.5 and 1.8.
Catalyst Activity
[0075] The catalytic activity of the samples was characterized for
CO oxidation using steady state conversion in a fixed bed reactor
(eight-channel fixed-bed reactor--Celero). A gas mixture containing
1100 ppm CO, 5% O.sub.2, and the remaining N.sub.2, was delivered
via mass flow controllers at a GHSV of 50,000 h.sup.-1. The gases
were passed through a pre-heater before contacting the catalyst
bed. Upon reaching the desired temperature, the reaction was
allowed to achieve steady state, and was held at that temperature
for 1 hour prior to sampling. The product gases were analyzed using
a gas chromatograph equipped with a methanizer and field ionization
detector (Shimadzu GC-17A). The conversion was calculated based
upon the CO/CO.sub.2 ratios in the inlet and product streams.
[0076] For the unwashed samples, 0.15 g of the nanocomposite were
placed in a 3 mm reaction well. For the washed samples, the amount
of nanocomposite was reduced to 0.05 g and the GHSV was increased
to 300,000 h.sup.-1.
[0077] The foregoing detailed description of the various
embodiments of the present teachings has been provided for the
purposes of illustration and description. It is not intended to be
exhaustive or to limit the present teachings to the precise
embodiments disclosed. Many modifications and variations will be
apparent to practitioners skilled in this art. The embodiments were
chosen and described in order to best explain the principles of the
present teachings and their practical application, thereby enabling
others skilled in the art to understand the present teachings for
various embodiments and with various modifications as are suited to
the particular use contemplated. It is intended that the scope of
the present teachings be defined by the following claims and their
equivalents.
* * * * *